reflectance standard is an opaque white glass plate with a finely ground surface having a reflectance of 0.856 relative to that of a freshly prepared magnesium oxide surface measured in place in the instrument. With the standard white plate in position in the light beam, the galvanometer is adjusted to read zero, the rheostat scale set to 85.6, the lamp turned on, and the shutter, B, moved until the galvanometer again reads zero. The standard plate is then replaced by the sugar sample by moving the slide. The galvanometer deflects and the circuit balance is restored by adjusting the rheostat. The scale reading then indicates the reflectance of the sample relative to magnesium oxide. A comparison of readings made by the authors for individual samples indicated the necessity of screening the sugars to uniform grain size.

For transmittancy measurements, a 150-g sample of sugar is dissolved in distilled water at room temperature and, without filtering is made up to 250 ml. The solution, free from air bubbles, is transferred to a 150-mm absorption cell with plane parallel end plates.

A reference cell, similar to, but only half as long as, the solution cell, is also filled with distilled water. The reasons for using the 75

FIGURE 75.-Compensating photoelectric circuit of Keane and Brice. Pi, Measuring photocell; P1, compensating photocell; G, galvanometer: R, potentiometer rheostat.

mm solvent cell are: (1) The sugar solution used contains only 51.2 percent of water; (2) water of this depth appreciably absorbs red light beyond about 690 mμ with a maximum absorption at 760 mμ; and (3) the red filter used, freely transmits light absorbed by the water, and the photronic cell shows appreciable response to radiation in this part of the spectrum. One starts with the 75-mm cell of water in the light beam and, with the standard white plate in position, the scale is set to read 100. After the circuit is balanced by turning screw B, the water cell is replaced by the 150-mm solution cell and when the circuit is balanced by means of the rheostat, the scale indicates the transmittancy of the solution. The transmittancies are measured first with a blue-green filter (Corning light shade blue-green, No. 428, 3.4 mm thick) and then with a red filter (Corning traffic red No. 245). The apparent color index is then computed from the ratio of the transmittancies by the formula

Turbidity measurement is further considered in chapter XX,*p. 341.

• Nees, [32] using a Lange photoelectric colorimeter [35], determines color and turbidity in the unfiltered solution with blue and yellow filters. The apparatus is first calibrated by reading the relative percentage of absorption of blue and yellow light by a given unit of color and turbidity. From this relationship both color and turbidity are calculated and expressed as the percentage of absorption of blue light.

(e) MERCURY-ARC SPECTRAL FILTERS

The mercury arc is useful, not only for the wave-length calibration of spectrometers, but because the intense light of three lines of the mercury spectrum may be isolated in sufficient purity by means of properly selected filters. It constitutes an admirable light source for abridged spectrophotometry of sugar solutions, as indicated above. Spectral filters for this purpose may be composed either of colored glasses, single and in various combinations, or of dyed gelatine film properly mounted. The composition of various glass spectral filters is to the found in the paper by Gibson, Tyndall, and McNicholas [8], and in the catalog of Corning Glass Works entitled "Glass Color Filters." The filters given in table 39 have been used both in visual photometry and with the photoelectric apparatus described under (c), p. 319. It is to be emphasized that when photoelectric cells are used, high spectral purity is essential, and since all of the filters transmit more or less red and infrared, this radiation must be removed from the radiation reaching the photocell. This may be done by interposing a water solution of copper sulfate of such thickness and concentration that the number of grams of CuSO4.5H2O per liter is equal to 178 divided by the length of the cell in centimeters. For visual photometry the red may be removed by means of Corning Dark Shade Blue-Green No. 430, 4 mm thick. In table 39 are given the mercury wave length isolated, the designation of the glass components, the thickness, and the transmission of each of the filters of this particular set at the wave length isolated. There is also included a list of dyed gelatine filters under the manufacturer's designation. These are described in the catalog of the Eastman Kodak Co., Rochester, N. Y., under the title Wratten Light Filters. These filters also transmit some visible red.

Several subvarieties of both serpentine and amphibole are referred to in the literature as being suitable for laboratory filter aids. A large proportion of long fibers is desirable for the filtration of sugar sirups and the variety chosen should be resistant to the drastic chemical treatment described below, the purpose of which is to remove estremely fine material that may cause cloudy filtrates, and other substances that might affect the color of the solutions. Hard columnar fiber

bundles (sometimes 20 cm or more in length) should be broken to a length of 3 or 4 cm and loosened sufficiently to allow easy permeation of liquid. The asbestos fiber, in a suitable iron container such as a sand bath, is treated with a solution of sodium hydroxide, specific gravity 1.43 (40-percent NaOH), 250 ml to each 25 g. The vessel is covered and the mixture is boiled for 30 minutes with occasional stirring, no attempt being made to maintain the above concentration. The mixture is filtered by suction in a Büchner funnel without paper and washed with relays of clear tap water until substantially all alkali is removed from the filter pad. The washed asbestos, after pressing in the Büchner funnel, is transferred to a glass flask and treated with a mixture of 250 ml of hydrochloric acid, specific gravity 1.20, and 25 ml of nitric acid, specific gravity 1.42, for each 25 g of asbestos originally taken. The fiber pad is disintegrated and mixed with the acid by shaking and the mixture is heated for 30 minutes on the steam bath. The contents of the flask are then diluted with distilled water, filtered, and washed with hot distilled water until the washings give no reaction for acid or for chlorine ions. The fiber is dried at 100° to 110° C and stored in a clean container.

Asbestos for general analytical purposes has been prepared in the same manner. For the clarification of sugar solutions, three grades of long-fibered asbestos, designated consecutively according to fiber length as XXX, XX, and A 18 have been found satisfactory. Whether the asbestos is crude fiber or acid-washed, it should be subjected to the treatment described above for the removal of fines.

(b) ASBESTOS FILTERS

Several forms of filters are suitable for sirup filtration with asbestos. A 25-ml or larger Gooch crucible fitted with a disk of 200-mesh bolting silk to retain the asbestos is convenient and low-priced [40]. A good grade of filter paper (not hardened) may be substituted for the silk. Small Büchner funnels with filter paper also may be used. The size of the filter should be chosen with regard to the amount of solution to be filtered which, in turn depends upon the depth of color of the solution.

Jena glass filters [41] in the form of cylindrical funnels, 60- or 120ml capacity with 4-cm filter element, have proved very satisfactory. These filters are designated as 11-G-1 and 11-G-2 for the 60-ml capacity, and 11-a-G-1 and 11-a-G-2 for the 120-ml capacity. The final figure designates the pore size of the filter element, No. 1 being the larger. The pore size, No. 1, is used for preliminary filtration, and the No. 2 for the final filtration.

To form the filtering pad, the asbestos in water suspension is poured into the filter, sucked down with the aspirator, and packed tightly by pressing and tapping with a flattened glass rod. The flat pad, which should be about 5 mm thick when tightly packed, is then washed a few times with water and drained by suction.

Where many samples are to be run daily, a battery of filters may be arranged as shown in figure 76. Each filter or Gooch adapter is fitted to an 8-ounce wide-mouthed bottle through a two-hole rubber stopper, the other hole being fitted with a glass tee with a two-way (Geisler) stopcock, which leads to a vacuum header, and which in

"Powhatan Mining Co., Woodlawn, Baltimore, Md.

turn leads to a central chamber connected with the vacuum pump or water aspirator. Each suction tube passing from the filter to the vacuum header may be closed by means of the stopcock so that the filtering process for any individual unit may be interrupted without interference with the others. The central vacuum chamber is connected with a mercury manometer or other vacuum gage.

(c) PREPARATION AND FILTRATION OF THE SOLUTION WITH ASBESTOS [42]

A generous supply of solution should be available, the amount to be taken depending upon the depth of color. For darker products, 100 g of solution may be sufficient, whereas 200 g or more may be neces

sary for white sugars. The stability of transparency is, in large part, dependent upon high density. It is therefore considered necessary that the dry-substance concentration of the solution be not lower than 60 Brix, which is compatible with reasonably rapid filtration. Time is saved by dissolving and diluting on a weight basis. The required weights of sample and water are calculated as follows:

Then

g solution required XBrix of solution

Brix of sample

g sample to be taken.

g water to be added=g solution-g sample.

For nearly dry products of relatively high purity, such as 96 test raw sugars, a sufficiently close approximation results if polarization is substituted for Brix.

The calculated amount of sample to provide a 60-Brix solution is weighed into a tared flask on a rough balance and weights equivalent to the required water are added to the pan. A small amount of hot distilled water is added and the flask is placed in a water bath heated to 50° C and shaken to promote rapid solution. Hot water is added to the flask a little at a time until the sample is dissolved. The flask is dried on the outside and returned to the balance pan where dilution to the required weight may be completed. Purified dry asbestos is added to the solution in amount depending upon the quantity and character of the suspended matter present. Usually 0.5 g will suffice, but when slimy material is present and the solution is refractory in filtration, 1 to 2 g should be used. The flask is loosely stoppered and shaken gently at first until the warm air is expelled. The stopper is then tightened and the flask is shaken vigorously to mix the contents thoroughly and to permit the asbestos fibers and the suspended matter to become entangled. The flask is returned to the bath and the solution is allowed to warm for a short time.

Although it is possible to obtain a satisfactory filtrate with a single filtration, particularly in the case of easy filtering solutions, it is customary to filter twice. Two filters are therefore prepared as directed in section (b), p. 325. The first, or preliminary filter, immediately before filtration, is warmed by washing with a small amount of hot distilled water, which is aspirated from the asbestos pad as thoroughly as possible. The warm solution is then added and a few milliliters filtered to displace the water remaining in the pad when the suction is closed off and a clean dry receiver is substituted. When the filtration is resumed the remainder of the solution, or as much as the filter will hold, is added. The pad is to be kept covered with solution during the filtration, at the end of which the suction is stopped before the pad becomes uncovered. The receiver is detached and returned to the bath for further warming while the second filter is being rinsed with hot water and drained. The second filtration is performed exactly as the first but no asbestos is added to the first filtrate. The main portion of the filtrate is collected in a clean receiver. This filtrate is cooled and adhering condensed water is wiped from the neck of the bottle which is then closed with a clean, dry stopper and shaken to mix the contents throughly. The refractometric Brix of the optical filtrate is determined and c (g dry substance per 1 ml) is obtained by reference to table 114, p. 632). The solution, if not too dark, is now ready for photometric observation.

(d) PREPARATION OF DIATOMACEOUS EARTH AND FILTRATION

Diatomaceous earth, also known as infusorial earth or kieselguhr, on account of its availability and ease of application, is preferred in factory laboratories for the clarification of sugar solutions for colorimetry. As noted by Balch [57], variable quantities of earth must be used to obtain satisfactory clarification, depending upon the nature and amount of suspended matter present as applying to different grades of sugar products. Zerban and Sattler [58] found that the same statement applies to products of a single grade (raw sugar), and that there is a gradual falling off in- logt in some cases with increasing quantities of earth used, and that more earth than the quantities tested (5 g to 50 ml of 60 Brix) would have to be used to reach a limiting effect.